1. Field of the Invention
The present invention relates to an optical head which is capable of recording onto and playing back from optical disc media having differing substrate thicknesses, such as DVDs (digital video discs) and CDs (compact discs) in an optical disc apparatus.
2. Description of Related Art
In the above-noted type of optical disc apparatus which is generally used, to maintain the surface of the recording medium formed in the order of microns, an optical head is used which forms a minutely small spot onto the recording medium surface through a transparent substrate. In this type of configuration, because a collimated beam passes through a transparent substrate that is a parallel flat plate, a wavefront aberration occurs that is dependent upon the thickness of the parallel flat plate.
The term wavefront aberration used here refers to a skew in the collimated beam from an equiphase surface. If this skew is large, the size of the collimated spot expands to greater than the diffraction limit, preventing good-quality recording and playback characteristics.
Therefore, to form a minutely small spot that is near the diffraction limit onto the recording medium, a lens in the optical head optics system is designed so as to compensate for this wavefront aberration. Recently, it has become possible to include a correction function using a single aspherical lens.
The thickness of the substrate of an optical disc will depend upon the purpose thereof, such as with the substrates of an optical disk, for example, DVDs and CDs. When the substrate thickness varies, because the amount of wavefront aberration compensation varies, in a head that is designed to accommodate a substrate thickness for a specific type of optical disc, it is not possible to compensate for the wavefront aberration of a separate disc having a substrate of a different thickness, making it impossible to perform recording and playback of that type of disc.
To solve this problem in the past, there was a technique proposed whereby the distance between the laser light source 1 and the objective lens 2 was changed, as shown in FIG. 4, thereby enabling a reduction of aberration caused by the difference in substrate thickness.
In this example, the light radiated by the laser light source 1 which is disposed at a first position P1 passes through the objective lens 2, and is collected onto the first optical disc 3. In the case of a second optical disc 4 which has a different substrate thickness, the laser light source 1 is moved to a second position P2 at which the aberration is minimum for the substrate thickness of this optical disc 4, thereby enabling optical disc playback with a minutely small spot.
However, in the above-noted optical head, when moving the laser light source 1 to the second position P2 at which the aberration is minimum with respect to the optical disc 2, because of error in the movement position thereof, the aberration characteristics can actually be worsened.
More specifically, FIG. 3 shows the relationship between the change in the position of the laser light source and the allowable amount of overall wavefront aberration and shift (lateral skew to the outside of the light axis and is referred to a lateral skew hereunder), in an optical system including the objective lens. From this drawing, it can be seen that, accompanying a change in the position of the laser light source, although there exists a position at which the overall aberration is minimum, this is accompanied by a reduction in the allowable lateral shift.
For this reason, when the laser light source 1 moves to the second position P2 in FIG. 4, although the associated aberration is minimum, the reduction in the allowable error under this condition in fact causes a sharp worsening of the aberration characteristics.
In view of the foregoing, an object of the present invention is to provide an optical head which solves the above problem, and which can accommodate optical discs of different substrate thicknesses without a worsening of the aberration characteristics.